Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer and comonomer into a gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst becomes inactive or the polymer particles become too sticky and the bed commences to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent such as nitrogen and/or one or more alkanes (e.g. one or more of propane, butane, pentane, hexane, heptane, octane) and/or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
The man skilled in the art knows that some liquid could advantageously be used in the polymerisation fluidised bed technology in order to assist in the heat removal within the polymer bed and correspondingly increase the production rate. This can be operated by cooling the recycle gas below its dew point so that liquid is introduced into the reactor through the recycle process.
For example, in EP89691 and EP0241947, an inert liquid may be introduced into the recycle stream to increase its dew point. The resulting ability to remove greater quantities of heat energy in less time has increased the production capacity of the typical exothermic fluidised bed reactor. It is recited that the entry point for the two-phase recycle stream should be below the fluidised bed (polymerisation zone) to ensure uniformity of the upwardly flowing gas stream and to maintain the bed in a suspended condition. It is also recited that no noticeable temperature gradient appears to exist within the upper portion of the bed whilst a temperature gradient will exist in the bottom of the bed in a layer of about 6 to 12 inches, between the temperature of the inlet fluid and the temperature of the remainder of the bed.
WO9428032 discloses the separation of at least part of the condensed liquid from the recycle gaseous stream and its introduction directly into the fluidised bed at or above the point at which the gaseous stream passing through the fluidised bed has substantially reached the temperature of the gaseous stream being withdrawn from the reactor. Point or points of introduction of the liquid into the fluidised bed located at approximately 50-70 cm above the fluidisation grid are recommended.
It recites that commercial processes for the gas fluidised bed polymerisation of olefins are generally operated under substantially isothermal, steady state conditions. However, although at least a major portion of the fluidised bed is maintained at the desired substantially isothermal polymerisation temperature, there normally exists a temperature gradient in the region of the bed immediately above the point of introduction of the cooled recycle gaseous stream into the bed. The lower temperature limit of this region wherein the temperature gradient exists is the temperature of the incoming cool recycle gas stream, and the upper limit is the substantially isothermal bed temperature. In commercial reactors of the type which employ a fluidisation grid, this temperature gradient normally exists in a layer of about 15 to 30 cm (6 to 12 inches) above the grid.
In WO9425495, WO9425497, WO9610590 and WO9610591 (“Dechellis”), whilst it is claimed that higher condensation rates can be obtained, it is also recited that fluidised bulk density (FBD), and particularly the ratio of fluidised bulk density to settled bulk density (SBD), are asserted to be limiting factors for stable operation where higher quantities of liquid are used in the recycle stream. The entry point for the recycle stream is said to be preferably below the fluidized bed so as to provide a uniform flow of the recycle stream to maintain the fluidized bed in a suspended condition and to ensure uniformity of the recycle stream passing upwardly throughout the fluidized bed.
Some of the examples disclosed in WO9425495 show very high condensation rates obtained thanks to the control of the ratio of fluidised bulk density to settled bulk density, the polymerisation process being otherwise operated under conventional operations, i.e. including the recycling of the gas/liquid flow below the fluidisation grid.
U.S. Pat. No. 6,391,985 discloses a method of achieving and utilizing a high percentage of liquid in the recycle by deliberately adjusting the conditions in the reactor to pass from the bubbling mode of fluidization to turbulent fluidization and increasing the condensing level (the amount of liquid introduced through recycle) to a desired level of 17.5% or higher, preferably 20% or higher.
It is also recommended therein to utilize a ratio of fluidized bulk density to settled bulk density (FBD/SBD) less than 0.59, i.e. exactly the opposite teaching compared to Dechellis.